Process and system for depolymerizing plastic

Abstract

A continuous flow process and system for depolymerizing plastic. A heterogeneous mixture of solid plastic particles, a solvent, and a catalyst are pumped continuously through a heating zone at a flow rate resulting in a particle speed sufficient to keep the plastic particles in suspension. The heterogeneous mixture is heated in the heating zone and maintained in a hold zone to complete depolymerization of the mixture into a homogeneous solution containing a liquefied reaction product. The homogeneous solution is cooled to solidify and precipitate a solid reaction product. The solid reaction product is separated from the solvent to be recycled. Contaminants are removed from the solvent, and the solvent is recirculated for use as a constituent of the heterogeneous mixture.

What is claimed is: 
     
         1 . A continuous flow process for depolymerizing plastic, comprising:
 a) continuously flowing a mixture containing solid plastic particles in a solvent and in the presence of a catalyst through a line in a heating chamber at a particle speed sufficient to maintain suspension of the plastic particles in the solvent and to prevent the plastic particles from agglomerating and clogging the line; wherein the plastic particles consist of polyethylene terephthalate, modified polyethylene terephthalate, polyethylene terephthalate blends, polyethylene naphthalate, polybutylene terephthalate, polyethylene terephthalate glycol, polylactic acid, poly (glycolic acid), poly D,L-lactic-co-glycolic acid, polyethylene 2,5-furandicarboxylate, and copolyesters, or any combination thereof; wherein the solvent consists of ethylene glycol, diethylene glycol, glycol ethers, 2-ethyl hexanol, tetramethyl cyclobutanediol, cyclohexanedimethanol, alcohols, ethanol amine, ionic liquids, polar protic solvents, polar aprotic solvents, or any combination thereof;   b) transferring heat through the line in the heating chamber to heat the mixture to a reaction temperature to start the depolymerization of the plastic particles in the solvent into a homogeneous solution including a liquefied reaction product; wherein the reaction temperature is within 20° C. of the melting point of the plastic particles in the heterogeneous mixture; wherein the liquified reaction product includes monomers, dimers, or oligomers; and   c) holding the mixture at the reaction temperature for at least one minute.   
     
     
         2 . The continuous flow process of  claim 1 , wherein the liquefied reaction product includes (bis (2-hydroxyethyl) terephthalate, dimethyl terephthalate, terephthalic acid, (bis (2-hydroxyethyl) naphthalate, (bis (2-hydroxyethyl) Furanoate, their respective oligomers, acids, half-esters, mixed esters, dioctyl terephthalate, diisobutyl terephthalate, dibutyl terephthalate, bisphenol A, lactates, bis (2-hydroxyethyl) terephthalamide, other terephthalamides, or any combination thereof. 
     
     
         3 . The continuous flow process of  claim 1 , wherein the reaction temperature is at least 150° C. 
     
     
         4 . The continuous flow process of  claim 1 , wherein step (b) further includes holding the mixture in a hold tube at the reaction temperature for at least one minute. 
     
     
         5 . The continuous flow process of  claim 1 , wherein step (a) further includes preheating the mixture in a preheating heat exchanger before flowing it into the heating chamber. 
     
     
         6 . The continuous flow process of  claim 5 , further comprising:
 c) flowing the homogeneous solution through a passage in the preheating heat exchanger after the homogeneous solution exits the heating chamber, wherein the homogeneous solution transfers heat to the mixture in the preheating heat exchanger.   
     
     
         7 . The continuous flow process of  claim 1 , further comprising maintaining a system pressure above the vapor pressure of the solvent at the reaction temperature to prevent the solvent from evaporating. 
     
     
         8 . The continuous flow process of  claim 1 , wherein the catalyst consists of zinc salts, zinc acetate, zinc chloride, titanium salts, titanium (IV) isopropoxide, titanium (IV) n-butoxide, manganese salts, magnesium salts, sodium hydroxide, potassium hydroxide, 1, 5, 7-Triazabicyclo [4.4.0] dec-5-ene, 1, 8-Diazabicyclo [5.4.0] undec-7-ene, magnesium acetate, 4-dimethylaminopyridine, amine, trialkyl amine, or any combination thereof. 
     
     
         9 . The continuous flow process of  claim 1 , further comprising:
 d) cooling the homogeneous solution in a chilling heat exchanger to a temperature below 50° C.;   e) settling the cooled homogeneous solution at room temperature for a time between about 0.5 hour and 100 hours to allow the liquefied reaction product to solidify into a solid reaction product;   f) separating the solid reaction product from the solvent by one or more of decanting, filtering, centrifuging, pressing, and distillation; and   g) reusing the solvent separated from the solid reaction product in the process.   
     
     
         10 . The continuous flow process of  claim 1 , further comprising:
 c) separating the solvent from the reaction product;   d) removing contaminants from the solvent by filtration or with sorbents; and   e) reusing the solvent in the process by mixing solid plastic particles in the reused solvent to form the mixture.   
     
     
         11 . The continuous flow process of  claim 1 , wherein the solid plastic particles comprise polyethylene terephthalate; and wherein the liquefied reaction product comprises:
 85% to 98% of bis(2-Hydroxyethyl) terephthalate;   less than 10% of 2-hydroxyethyl terephthalic acid;   less than 10% of dimers;   less than 6% of mixed esters with diethylene glycol;   less than 1% of terephthalic acid;   less than 1% of trimers; and   less than 1% of higher oligomers.   
     
     
         12 . A system for continuous depolymerization of plastic, comprising:
 a pump operating at a flow rate;   a line through which the pump continuously feeds a heterogeneous mixture including solid plastic particles in a solvent in the presence of a catalyst at a particle speed; wherein the plastic particles consist of polyethylene terephthalate, modified polyethylene terephthalate, polyethylene terephthalate blends, polyethylene naphthalate, polybutylene terephthalate, polyethylene terephthalate glycol, polylactic acid, poly (glycolic acid), poly D,L-lactic-co-glycolic acid, polyethylene 2,5-furandicarboxylate, and copolyesters, or any combination thereof; wherein the solvent consists of ethylene glycol, diethylene glycol, glycol ethers, 2-ethyl hexanol, tetramethyl cyclobutanediol, cyclohexanedimethanol, alcohols, ethanol amine, ionic liquids, polar protic solvents, polar aprotic solvents, or any combination thereof;   a heating chamber raising the temperature of the heterogeneous mixture flowing through the line to a reaction temperature, wherein the reaction temperature is within 20° C. of the melting point of the plastic particles in the heterogeneous mixture; wherein the conversion of the heterogeneous mixture containing the solid plastic particles into a homogeneous solution containing a liquefied reaction product is started in the heating chamber; and   a hold tube receiving the heated heterogeneous mixture from the heating zone to maintain the reaction temperature for a hold time of at least one minute.   
     
     
         13 . The system of  claim 12 , further comprising:
 a mixer upstream of the heating chamber using an agitator or recirculating solvent to stir the heterogeneous mixture.   
     
     
         14 . The system of  claim 12 , wherein the hold tube is an insulated pipe or tubing having a length sufficient to ensure complete conversion of the heterogeneous mixture into the homogeneous solution containing liquefied reaction product. 
     
     
         15 . The system of  claim 12 , wherein the hold time in the hold tube is between 1 minute and 60 minutes.